Proposal (Trac ticket #3671): Add takeRec, genericTakeRec and spanRec to Data.List

[Philip K.F. Hölzenspies]

On Thu, 2009-11-19 at 13:28 +0200, Yitzchak Gale wrote:
> Philip K.F. wrote:
> > runs :: (a -> a -> Bool) -> [a] -> [[a]]
> > runs p xs = ...
> >
> > which produces a list of runs, i.e. the first result is that prefix of
> > xs, such that for all consecutive elements e_i, e_{i+1}, the property
> > holds, i.e. p e_i e_{i+1} -->> True.
> 
> We already have something like that:
> 
> groupBy :: (a -> a -> Bool) -> [a] -> [[a]]

The groupBy function compares the first element e_1 to all consecutive
elements e_i until it finds one for which the predicate doesn't hold.
The runs function compares *consecutive* elements e_i and e_{i+1}. By
example:

> runs (<) [1,2,3,4,3,4,5]
[[1,2,3,4],[3,4,5]]


> In fact, instead of spans and breaks, why not just use:
> 
> runs :: (a -> Bool) -> [a] -> [[a]]
> runs = groupBy . on (==)

groupBy . on (==) :: (a -> ()) -> [a] -> [[a]]

> Then we have:
> 
> breaks p = runs p . dropWhile p
> spans p = runs p . dropWhile (not . p)

For spans and breaks there might be a composition of functions already
in Data.List, but because of the above type check failure, it's not
this. Also, runs is more general than spans and breaks and runs is
actually more sorely missed. I may still be mistaken, but there are no
functions in Data.List that allow predicates on *consecutive* list
elements.

Regards,
Philip